In order to achieve a reduction of NOx components in exhaust gases, reduction-type catalytic converters have been developed, in particular for diesel engines, which are commonly subdivided into SCR catalytic converters (“selective catalytic reduction”) and storage-type catalytic converters. SCR converters are regenerated by supplying a urea-based reducing agent and/or an ammonia-based reducing agent, while storage-type converters are regenerated during rich exhaust phases by using the entrained hydrocarbons of the engine fuel.
A device, which meters urea as a reducing agent for the removal of nitrogen oxides from exhaust gases, e.g., from a diesel engine, is known from German Published Patent Application No. 199 46 900. Metering takes place by using a valve which lets through doses of urea which are determined by the electrical control of the metering valve, its throttle cross section, and the pressure difference applied to the throttle valve. The pressure upstream from the valve is measured and is kept constant within a tolerance range.